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When ffmpeg encodes a single large vob file it is fine but if I take VTS_01_1.vob and combine it with VTS_01_2.vob, VTS_01_3.vob, VTS_01_4.vob and VTS_01_5.vob using a file concatenation outside of ffmpeg then use ffmpeg to try to read the duration it is wrong. If I call ffmpeg -i on any of the vobs separately the duration and bitrate is correct. It seems to only be an issue when using several vobs combined together.

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ffmpeg is reading from the start and end of the file so possibly it is reading the pts and a rollover is occuring.